The present technology relates to control devices, control methods, and master-disc fabricating apparatuses. More specifically, the present technology relates to a control device, a control method, and a master-disc fabricating apparatus which are used to fabricate a master disc for information recording media.
Phase transition mastering (PTM) systems for performing thermal recording have been mainly used as systems for cutting Blu-ray Discs (BDs, registered trademark), which currently have the highest-density optical disc format. Because of the thermal recording, the PTM is suitable for fabricating optical discs that comply with a constant linear velocity (CLV) format. When a land/groove system is employed in order to further increase the density of optical discs, it is desired to perform disc cutting according to a zone constant angular velocity (ZCAV) format.
Heretofore, a system for performing cutting while rotating a master disc at a constant angular velocity (CAV) has been employed in optical-disc master fabrication employing the ZCAV format. FIG. 12 is a block diagram illustrating the schematic configuration of a master-disc fabricating apparatus employing such a cutting system. As illustrated in FIG. 12, this master-disc fabricating apparatus includes a formatter 101 and a cutter 102. The formatter 101 includes a clock switcher 111, a wobble-signal generator 112, a phase locked loop (PLL) 113, and dividers 114 and 115. The formatter 101 generates a wobble signal and supplies the wobble signal to the cutter 102. The cutter 102 generates a frequency-generator (FG) signal, which is a disc-rotation synchronization signal, and supplies the FG signal to the formatter 101. The master-master fabricating apparatus having such a configuration multiplies the FG signal by a PLL clock to perform clock multiplication switching, thereby realizing switching of zones in the ZCAV format.
However, use of the above-described system to perform ZCAV-format disc-master cutting in a PTM system has a problem. Since the master-disc fabricating apparatus having the configuration illustrated in FIG. 12 rotates a master disc by using a CAV system, the local linear velocity changes depending on the radial position, thus making it difficult to form a uniform groove shape. Although a method in which laser power is varied depending on the radial position is also conceivable, it is extremely difficult to set the laser power suitable for the CAV format so as to satisfy the cutting accuracy used for optical discs having a higher recording density than that of BDs. Thus, it is desirable to make it possible to perform ZCAV-format disc-master cutting, while maintaining the rotation of the master disc by using the CLV system.
In addition, in recent years, a variety of formats other than the ZCAV system have also been proposed as master-disc formats, and it is desired that master discs with those formats be also fabricated by the PTM system. It is also desirable that, in fabrication of master discs with those formats, the master discs can be cut while the rotation of the master discs is maintained by the CLV system, as in the case of master discs with the above-described ZCAV format.
As related technologies for addressing the above-described difficulties, some systems have been proposed which vary a formatter drive clock in accordance with a disc radial position by rotating a master disc by using the CLV system (see, for example, Japanese Unexamined Patent Application Publication Nos. 2007-134045, 6-36357, 2006-119484, and 7-287875). In practice, however, it difficult to cut a master disc with any of those systems, as described below.
When a reference clock frequency used for generating a clock used for a frequency divider is calculated using parameters of BD on the basis of the description in paragraph [0054] in Japanese Unexamined Patent Application Publication No. 2007-134045, about a value of 1019 Hz is yielded (P=0.32 μm, Nf=2024113, and VL=4.917 m/s). This indicates that implementation is extremely difficult with the currently available technology. The technology disclosed in Japanese Unexamined Patent Application Publication No. 6-36357 is also analogous in terms of generating a formatter drive clock by using a frequency divider, and thus suffers a similar problem to that of Japanese Unexamined Patent Application Publication No. 2007-134045.
Japanese Unexamined Patent Application Publication No. 2006-119484 discloses a technology using a direct digital synthesizer (DDS). A DDS digitally generates various output waveforms by using an output of a synchronization circuit that operates in accordance with a reference clock. When attention is given to a relationship between the frequency of the reference clock and the frequency of a signal that can be output, the DDS can also be regarded as one type of frequency divider. Although Paragraph [0025] in Japanese Unexamined Patent Application Publication No. 2006-119484 explains that the DDS of 100 MHz and 32 bits has a frequency resolution of about 0.023 Hz, this is a resolution at the lowest frequency that can be output. In order to realize a desired frequency resolution with the frequency of a reference clock used for an optical disc format, for example, with a frequency of about 66 MHz which is the frequency of a 1× speed channel clock in the case of BD, a very high reference-clock frequency is still necessary.
The technology disclosed in Japanese Unexamined Patent Application Publication No. 7-287875 concerns a system in which a voltage-controlled oscillator (VCO) is used to generate a formatter drive clock to be used. However, the VCO is a device that can provide a sufficiently accurate clock output when used in a feedback loop in a PLL. In Japanese Unexamined Patent Application Publication No. 7-287875, the VCO 5 illustrated in FIG. 1 is not feedback-controlled and is used alone. A frequency difference for each track on a BD is about 0.001% even at the inner circumference, and a desired clock accuracy is even higher than this value. With current technologies, it is yet extremely difficult without feedback control to realize a VCO that stably outputs a clock with such high accuracy in accordance with a digital-to-analog (D/A) output value.